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Temperature gradient solution zoning growth and characterization of ZnxCd1−xSe single crystals
386
Journal of Crystal Growth 70 (1984) 386—392 North-Holland, Amsterdam
TEMPERATURE GRADIENT SOLUTION ZONING GROWTH AND CHARACTERIZATION OF Zn~Cd1 ~Se SINGLE CRYSTALS -
A. BURGER and M. ROTH Graduate School of Applied Science and Technology, Hebrew University of Jerusalem, Jerusalem 91904, Israel
Single crystals of Zn~Cd1— ~Se with 0 ~ x ~ 0.3 have been grown from a high temperature Se solution using the TGSZ technique. 7 ohm cm) is analysis higher than thatand of CdSe ohm cm) grown by the same The Zn/Cd distribution has been measured using an original XRF system found crystals uniform (106 throughout any particular crystal. The darkdue resistivity Zn0 energy 3Cd07Se crystals (5 the x i0ternary compound. A discrete electron trapping level located 0.49 eV below the method, to the of larger band gap of conduction band with a trap density of 1.9 x 1011 cm has been attributed to a compensateddonor formed by an interstitial Cu impurity and a nearby Cd vacancy. The charge collection efficiency has been studied as a function ofvoltage applied to a thin Zn 0 3Cd0 7Se platelet. It is not limited, however, by the electron traps, but rather by the surface recombination of charge carriers. It has been suggested that an improved charge collection needed for the fabrication of (Zn,Cd) Se nuclear radiation detectors can be obtained by reduction of surface defects generated during the crystalline platelet processing.
1. Introduction The pseudobinary CdSe—ZnSe system is known to form a continuous series of solid solutions [1]. Single crystals of Zn~Cd1 ~Se have been grown from the vapor phase [2,31, and their optical properties have been investigated with respect to the application as laser screen materials in projection color 1’V. However, no information on the basic electrical properties of these ternary compounds exist in the literature. The charge transport properties of the Zn~Cd1 ~Se crystals are of particular interest, since the pure CdSe has been discovered recently [4,51as a novel semiconductor material suitable for fabrication of nuclear radiation detectors. In the present work we have grown for the first time single crystals of a ternary Zn0 3Cd07Se compound using the Temperature Gradient Solution Zoning (TGSZ) technique from the high temperature Se solution. This growth method has been chosen since in the case of CdSe [4] it yields relatively high dark resistivity (about 106 ohm cm) as-grown crystals. The vapor grown CdSe crystals, in contrast, exhibit much lower, typically 1 to 100 ohm cm, resistivity, which makes them unsuitable for lownoise room temperature operation as nuclear radia-
-
tion detectors [5]. Although thermal annealingofthe vapor grown CdSe in the Se2 vapor increase the resistivity by many orders of magnitude [5], the resulting reduction of the electron mobility, is due to the resistivity inhomogeneities, which degrade the charge transport properties of the material. The similarity of band structures and optical properties of CdSe and ZnSe [7], despite the difference in the crystal structure (hexagonal and cubic, respectively), suggests that the energy band gap (Eg) of the Zn~Cd1 ~Se must increase with the rising value of x. This trend has been experimentally verified for a complete series of solid solutions of a homologous Zn~Cd1 ~S compound [7]. Indeed, for the Zn0 3Cd0 7Se composition an experimental value of Eg = 2.00 eV has been determined [8], which is higher than Eg = 1.73 eV for CdSe (both measured at 300 K). Therefore, a lower dark current, beneficial for the detector operation, is expected at room temperature. The sufficiently high resistivity of the TGSZ grown Zn0 3Cd0 7Se crystals has allowed the study of their I—V characteristics, which have been analyzed in the framework of the space charge limited current (SCLC) theory [9]. The results have been compared with those obtained with the TGSZ CdSe crystals. Additionally, the charge collection efficiency in a thin
0022—0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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A. Burger, M. Roth
/
TGSZ growth and characterization of Zn~,,Cd
Zn0 3Cd07Se crystal has been studied using a standard nuclear measurements set up [10], and discussed in terms of the nuclear radiation detector performance.
2. Experimental 2.1. Single crystal growth The experimental set up used for the growth of single crystals was similar to that described previously for the TGSZ of CdSe. The starting materials were Alfa 5N CdSe and 5N ZnSe and the Aldrich 6N Se shot. The CdSe and ZnSe powders were further purified by sublimation in vacuum at 900 and 10000 C respectively. They were subsequently milled, mixed and sintered at 9000 C for 36 h. Typically 30 g charges were prepared with addition of 10 at% Se. Since neither the ternary nor the pseudobinary CdSe—ZnSe phase diagrams have ever been published, the liquidus temperature for the desired composition was determined experimentally using a conventional DTA set up. A 5 g charge of the Zn0 3Cd0 75e sintered compound with additional 10 at°,/0Se had been loaded into a 1 by 5 cm quartz tube and sealed under 10-i Torr vacuum. A similar reference tube with 3 g of Al203 powder had been prepared. Two identical Pt/Pt— 13% Rh thermocouples were inserted into wells made at the bottom ends of the sample and reference tubes. The differential temperature recording was carried out using a double pen Kipp and Zonen recorder directly, with a 20 1tV full scale. The thermal arrest temperature of the liquid (melted sample) was determined on slow cooling C/min), which provided the necessary close to equilibrium conditions of the experiment. The liquidus temperature of the Zn0 3Cd0 7Se + 10 at % Se composition was found to be 1120°C with a reproducibility of ±2°C. Therefore, prior to crystal growth the melt was soaked at a temperature of 1150°Cfor 24 h. During the crystal growthrun the ampule was lowered in a temperature gradient of 20°C/cmat a rate of 1 cm/day. Using these growth conditions, 1.5 cm diameter and several cm long single crystals of Zn0 3Cd0 7Se could be routinely produced. (30
3— ~Se
387
2.2. Sample preparation Slices with thicknesses varying from 0.2 to 2 mm were either cut (using a wire saw) or cleaved from the single crystal ingots.They were further thinned down through mechanochemical lapping and polishing in a 3 bromine—methanol solution. Additional etching in a 10 0/0 bromline—methanol solution followed by rinsing with pure methanol and triply distilled water was carried out before the application of contacts. Good ohmic contacts, necessary for the observation of space charge limited currents, could be produced using the In-Hg eutectic alloy. In the case of nuclear measurements blocking contacts were applied, usually by painting the carbon based Aquadag solution, though evaporated gold electrodes produced similar results. 0/~
2.3. Composition analysis The axial and radial Zn/Cd distributions were studied using a specially designed X-Ray Fluorescence (XRF) analysis system shown schematicallyin fig. 1. A 5 mCi gamma-ray point source of 125J served to excite the characteristic X-ray emission lines of Zn(Kct, 9.6 keY), Cd(Kcç 23 keV and KfI, 26 keY) and Se(Kc~,11.1 keY). The X-ray radiation spectrum was registered using an 8% energy resolution Hg12 detector [11] and displayed on the screen of an ORTEC Model 7100 multi-channel analyzer. A commercial Link XRF analysis system in conjunction with the JEOL JSM 35 scanning electron microscope were used for initial calibration of the composition of the Zn0 3Cd0 7Se crystal. A small sample was cut from the bottom part of the crystal for the purpose of calibration, and the Zn/Cd ratio was found to correspond exactly to the molar ratio of ZnSe and CdSe charges introduced into the growth ampoule. 3. Results and discussion The maximal concentration of ZnSe in the Zn~Cd1 Se crystals grown in the present work corresponded to x = 0.3. The growth of crystals of composition with x> 0.3 was limited by the equipment used. A photograph of a typical Zn0 3Cd0 7Se crystal grown by the TGSZ method is shown infig. 2 -
388
A. Burger, M. Roth
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TGSZ growth and characterization of Zn~Cd,- ~Se
HV Power Supply
Crystal
Hgli Detector
~
125)
~ Preamplili!jj Amplltier
L{
MCA
Source
Fig. I. X-ray fluorescence analysis system based on the Hg1
2 nuclear radiation spectrometer.
Fig. 2. Photograph of a single crystal of Zn0 3Cd0 7Se (left) grown by the TGSZ technique and the polycrystalline residue (right) of the starting materials.
together with the polycrystaffine residue of CdSe, ZnSe and Se remaining in excess after the crystallization of Zn0 3Cd07Se. As in the case of CdSe TGSZ growth [4] the homogeneity of the crystal has been achieved by lowering the ampoule through the temperature gradient existing in the furnace, which has kept the growth interface at a constant temperature. The lowering speed is limited by the solution zone motion speed, or by the upward Se diffusion rate in the molten zone. It has been verified experimentally that only polycrystalline boules of Zn0 3Cd0 7Se could be produced at lowering speeds faster than 1 cm/day. The crystal shown in fig. 2 was cut with a wire saw and the Zn/Cd atomic ratio was measured using the Hg12 XRF spectrometer system described in the previous section. The surprising result, which is very important for future applications of the mixed (Zn, Cd) Se crystals, was that both axial and radial ZnSe/CdSe molar ratio’s were found constant within the statistical error of 0.6%. This suggests that the solubilities and the diffusion rates ofZnSe and CdSe
in the Se solvent are very similar. The absence of any apparent segregation in the process of solidification may also indicate that the solidus—liquidus tie-lines of the Zn—Cd—Se system correspond to constant Zn/Cd ratios, but any certain confirmation of this hypothesis can come only from future studies of the ternary phase diagram. Another result observed using the LINK XRF analysis system, i.e. that the Zn/Cd ratio in the crystal corresponds exactly to the molar ratio of ZnSe and CdSe introduced into the feed material, indicates that the solidus—liquidus gap of the ZnSe—CdSe pseudobinary system is very small, at least for the Zn0 3Cd0 7Se composition. It is well known that CdSe crystallizes in the wurzite (hexagonal) structure, while ZnSe in the zincblende (cubic) structure. The structure of the solid solutions, Zn~Cd1 ~Se, depends on the cornposition of the compound. It has been found to the hexagonal for 0 ~ x ~ 0.5, as follows from the X-ray diffraction analysis performed by Budennaya et al. [3]. This result has been confirmed by our own X-ray diffraction measurements carried out using -
A. Burger, M. Roth
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TGSZ growth and characterization of Zn~Cd,— ~Se
the Zn03Cd07Se crystals. We have found that the crystals can be easily cleaved along the (1120) planes.samples In some of wire sawn, havecases been cleaved, prepared instead for the electrical measurements. The TGSZ grown Zn 0 3Cd0 7Se crystals exhibit a relatively high dark resistivity. A mean value of = 5 x iO~ohm cm has been obtained using the four point probe method [12]. Astointhethelack case of CdSe [4] the high resistivity is due of Se vacancies in crystals grown in an environment of excess Se. TheresistivityoftheZn 03Cd07Secrystals is high enough to use the SCLC theory [13] for the analysis of the charge transport properties of the crystals. Fig. 3 shows a typical I—V curve taken with a 1.8mm thick Zn02 3Cd0 sample at room temperaarea7Se metal contacts have been ture. Three applied to themm crystal, one of them being ohmic. The current rises linearly at low voltages, up to about 30 V. At higher voltages a clear quadratic increase of the current is observed, indicating that a discrete trap level lunits the current flow in this region. A steepnse of the current corresponding to the trap-filled-limit
389
(TFL) regime occurs at VTFL = 600 V. The narrow superquadratic transition region between the square 2) law behaviorand the onset ofthe TFL is not (Icc V extended enough to claim that a “diffused” rather than a discrete trap level is involved. The density of traps (N8) for one-carrier injection can be derived from a simple relation given by the SCLC theory: 2N ‘2 1 ~TFL ea ~, where e is the electron charge, a is the sample — —
thickness and t~is the static dielectric constant [7]. The dielectric constants of CdSe and ZnSe are very similar and a mean value of ~ = 9 is adopted for the present calculations. Thus, a value of 1.9 x 1011 cm ~ obtained forthe trap density (NJ. trap level energies (P28) have beenthe derived from theThe experimental I—V curve, since quadratic region follows Child’s law [9]: -
‘
—
8
A
£1T7213
lieU V
where A is the contacxt area, ~i is the electron (for an n-type crystal) mobility and 0 is the ratio of the free electron density to the trapped electron density in the crystal: ,
0
=
n/n 8
=
(N0/gN8) exp(—E8/kT),
(3)
where N0 is the effective density of states in the conduction band and gis the degeneracy factor equal to 2 for the electron traps [14].
:
The values of /ie and N0 are material dependent, and they are not known either for Zn0 3Cd07Se nor for the zincblende structure ZnSe. Therefore, s’ and 3 of pure CdSe2Y~ are taken for N the = 720 cm 0 =approximate 1.15 x 1018 evalucm— ation of the trap level energies. The value of P2 8 = 0.49 eV has been obtained by solving eqs. (2) and (3) and using the parameters described above. Table 1 summarizes the electron trap parameters of
10 3 .. 102
•.
/
101 Table 1 Trapping levels and concentrations 100 100
I
101 V
I
(V) 102
3) I
io~
2. taken with Fig. 3. TypicalI—V curve ofthe Zn0 3Cd07Se crystal a 1.8 mm thick platelet and contact area of 3 mm
Crystal TGSZ Zn
E,(eV)
N8(cm
0 3Cd0 75e 0.49 1.9 x 10” _____________________________________________________ TGSZ CdSe [4] 0.51 1.0 x 1012
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A. Burger. M. Roth
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TGSZ growth and characterization of Zn~Cd,- ~Se
the TGSZ grown Zn0 3Cd07Se and the TGSZ grown CdSe crystals for comparison. Both the trap level energies and the trap densities are apparently similar, especially in view of the rather approximate nature of our calculations, and of course, the differences in the crystal compositions. Manfredotti et al. [15] have also reported on the existance of an electron trap with an energy of 0.49 eY below the conduction band and density of 3 x 1011 cm in CdSe crystals with a mean dark resistivity of 108 ohm cm at room temperature. It is interesting to as-grown, note that medium this kindresistivity of trap crystals usually appears in the and we have attirbuted it to the formation of Cu impurity induced Frenkel-type defects, namely Cu 1—Vcd pairs. It would be, therefore, interesting to grow single crystals of CdSe and CdSe—ZnSe solid solutions by the TGSZ method using Cu free starting materials. The latter must be sinthesized from the constituent elements, since Se is known to be the major source of the Cu impurity. Preliminary experiments show that copper can be extracted from Selenium very effectively using the zone-refming method. Although the TGSZ growth minimizes the amount ofSe vacancies, a small amount ofvacancies remain in the crystals and they are responsible forthe residual n-type conductivity of the TGSZ grown Zn0 3Cd07Se and CdSe [4]. The n-type conductivity associated with the Se vacancies [6] is much lower in the TGSZ crystals than in the crystals grown from the vapor phase using a stoichiometric starting material [5], since the former are grown in a Se-excess environment. High temperature Se annealing of CdSe crystals [16] or vapor growth in the Se-excess atmosphere [17] can further reduce the number of Se vacancies, or donor levels, converting the crystals into nearly intrinsic or even p-type conductive. In other words, the resistivity of the crystals is closely related to the amount of Se vacancies present. It is, therefore, surprising that the TGSZ grown Zn0 3Cd0 7Se crystals exhibit a higher resistivity than the CdSe crystals grown by the same method. The opposite could be expected, since the Zn0 3Cd0 7Se crystals are grown at a temperature higher by several tens of degrees, allowing for a higher concentration of Se vacancies to be formed. The simplest explanation that can be given ~
is that the net concentration of ionized donors (ND NA) is much smaller than the concentration of thermally induced electrons. Thus, the resistivity depends mostly on the energy bandgap of the material, being higher for Zn0 3Cd0 7Se due to the larger bandgap of this material. As a consequence, lower leakage current and thus, lower electronic noise detectors (in comparison with CdSe) can be fabri. cated from the TGSZ grown Zn0 3Cd0 7Se crystals. This has been proven experimentally in a nuclear measurement using electronic system shown in 55Fean(5.9 keY) gamma point source fig. 1, butofwith instead the asample. The 5.9 keY gamma-peak could be clearly resolved from the noise threshold, which was impossible before with CdSe detec—
tors [4]. A detailed description of the Zn0 3Cd07Se detector performance will be given elsewhere [18], however some results of the nuclear measurements concerning the material dependent parameters are reported here. In a nuclear measurement carried out using a semiconductor detector gamma-rays (or X-rays and charged particles in other cases) interact with the crystal by converting most of their energy into dcctron—hole pairs. The charge carriers are collected by an applied electric field (E), and the resulting current pulse is proportional to the radiation energy. Usually, the signal is amplified and recorded as a histogram of the observed radiation events giving the number ofevents in each energy interval. In our case, the histograms or spectra, have been recorded using a multichannel analyzer. Thus, the total charge collected at the electrods is:
Q
=
~ iN~,
(4)
where i is the channel number and N~is the number of events in the ith channel. Q is a complex function of the mean drift length 2 = /iEE, where ji is the mobility and t is the trapping time of the charge carriers. At high electric fields the collected charge reaches the saturation value Q0, and the charge collection efficiency ~ for a uniform field * and the
*
Since the Zn0 3Cd0 7Se as well as the CdSe detectors never show the polarization effect, no space charge is beingbuilt up in the detector, which might alter the internal electric field.
A. Burger, M. Roth
/
TGSZ growth and characterization of Zn~Cd
1- ~Se
electron transport only* is given by the modified Hecht equation [10]: =
+
=
Qo
\
—~_i ~-1~-~ [~ exp(~T_~l, —
lieEJ
L
L
\lietE’JJ
(5) where L is the thickness ofthe detector and S is the carrier Q and Qsurface recombination velocity. The values of 0 are determined from the experiment using eq. (4). The experimentally dependence of the collection efficiency on obtained the magnitude of the applied electric field is shown in fig. 4. The striking result here is that the full charge collection is achieved at very high fields, above 4 x io~ V/cm. Even with mercuric iodide [10] having a much lower electron mobility, weak fields are needed for the full charge collection. There are two major factors that may limit the charge collection in the present case, namely the surface recombination and the bulk trapping of the charge carriers. A convenient estimate of the parameters involved can be made using the haif-maximum-collection value ofthe electric field (Ehmc). The latter can be derived from eq. (5), for ~ = 1/2, and is given by Eh mc
=
S//ie
+
(0.63//i e t)L.
(6)
___________________________________________ 100
A A
~
80 60
A A
A
A
A
A
A
40
A
20 A
A
00
3 (V/cm) Fig. 4. Charge collection ELECTRICefficiency FIELD as -x10 a function of the applied
391
We know from the SCLC analysis of the I—V curves described above that a single 3kindexists of trap in with our aZn density N8 = 1.9 x lO~cm 03Cd07Se crystals. The trapping time (t) can be then evaluated from the well known relation [19]: r= (NtaeVth)’, (7) where V~his the thermal velocity electrons being 0e is the of electron trapping equal to —~107 cm/s and cross section. Assuming even the largest cross section ever found in compound 2, we obtain t semiconduc5 x 10-6 s. tors [19], i.e. 1013 cm Using this value together with lie = 720 cm2 Y ‘ s 1, L = 0.01 cm (thickness of the detector used in this experiment) and Ehmc = 1600 V/cm (from fig. 4), we now calculate the carrier surface recombination velocity S = 106 cm/s. Comparing the calculated value of S with the much smaller ratio L/r = 2 x 10~cm/s, it is clear from eqs. (5) and (6) that the charge collection efficiency depends entirely on the surface recombination kinetics of the carriers. Even for several mm thick detectors instead of the 0.1 mm thick one used in the present work the condition L/z ~ S would be valid. The origin of the high carrier surface recombination velocity is still unclear. The most probable hypothesis is, however, that a high concentration of surface defects is introduced by the surface treatment during the detector fabrication. It is evident that no bulk defects ofthe Zn 0 3Cd0 7Se crystals play a role in the observed phenomenon. The charge collection properties of the CdSe crystals have not been studied before [4], since it was impossible to extract the 5.9 keY gamma-ray peak from the dcc‘~
—
.
tronic noise background. However, a similar surface damage in the fabrication process of the CdSe detectors occur.defects It is, therefore to study the may surface of both plausible CdSe and Zn~Cd 1 ~Se crystals in correlation with the surface —
electric field measured 55Fe (5.9 with keY) a 0.1gamma-ray mm thick point Zn0 3Cd07Se source through crystal irradiated the negative with electrode. a *
The penetration depth of the 5.9 keY gamma radiation for the Zn 0 3Cd0 7Se crystal is only 3—4 ~m which is much smaller than the detector thickness used in this work. Therefore, the contribution of the hole transport induced charge is negligible, when the polarity of the irradiated electrode is negative,
processing. 4. Conclusion
Large single crystals of the Zn~Cd1 ~Se ternary compound with x up to 0.3 have been grown usmg the TGSZ technique. Both the axial and radial distri—
392
A. Burger, M. Roth
/
TGSZ growth and characterization of Zn~Cd
butions of the cation have been found uniform throughout the crystals provided that the growth interface is kept at a constant temperature. The TGSZ growth based on crystallization from Se excess solution yields high dark resistivity 5 x l0~ohm cm) Zn0 3Cd0 7Se crystals aJmost free of Se vacancies, which are responsible for the n-type conductivity. The resistivity has been found to depend mostly on the thermally generated carriers. Therefore, the resistivity of Zn0 3Cd0 7Se crystals is higher by an order of magnitude than that of the CdSe, due to a larger energy bandgap. Lower leakage current and thus, lower noise detectors can be fabricated from these crystals. The electron transport in the Zn0 3Cd07Se crystals has been found to be limited by a discrete trap level located at 0.49 eV below the conduction band. The nature of the trap level is found to be similar to that observed in the CdSe crystals, and is related to the production of pairs including a Cu interstitital impurity (donor) compensated by a Cd vacancy. However, the performance of Zn~Cd1 ~Se crystals as nuclear radiation detectors has been found not to be limited by the bulk transport properties of the charge carriers, but rather by the high carrier surface recombination velocity originating probably from surface defects. (‘—j
—
1- ,~Se
References [1] S. Forgue, R. Goodrich and A. Cope, RCA Rev. 12 (1951) [2] Reimers, Phys. Status Solidi 35 G.S. (1969)Pekar 707. and G.N. [3] P. L.D. Budennaya, A.!. Nizkova, Polisskii, Inorg. Mater. 18 (1982) 760. [4] A. Burger and M. Roth, J. Crystal Growth 67 (1984) 507. [5] A. Burger, I. Shilo and M. Schieber, IEEE Trans. NucI. Sci. NS~30(1983) 368. [6] AL. 5280. Robinson and RH. Bube, J. Appl. Phys. 42 (1971) [7] H. Hartman, R. Mach and B. Selle, in: Current Topics in Materials Science, Yol. 9, Ed. E. Kaldis (North-Holland, Amsterdam, 1982). [8] Y. Brada, private communication. [9] M.A. Lampert, Phys. Rev. 103 (1956) 1648. [10] A. Levi, M. Schieber and Z. Burshtein, J. Appl. Phys. 54 (1983) 2472. [11] J. Nissenbaum, A. Holzer, M. Roth and M. Schieber, Advan. X-Ray Anal. 24 (1980) 303. [12] H.H. Weider, in: Non-Destructive Evaluation of Semiconductor Materials and Devices, Ed. J. Zemel (Plenum, New York, 1979). [13] MA. Lampert, Phys. Rev. 103 (1956) 1648. [14] J.S. Blakemore, Semiconductor Statistics (Pergamon, Oxford, 1962). [15] C. Manfredotti, A. Rizzo, L. Vasanelli, S. Galassini and L. Ruggiero, J. AppI. Phys. 44 (1973) 5463. [16] M. Itakura and H. Toyoda, Japan. J. Appl. Phys. 4 (1965) 560. [17] R. Baubinas, Z. Janushkevichus and A. Sakalas, Mater. Res. Bull. 8 (1973) 817. [18] A. Burger, M. Roth and M. Schieber, in: Proc. 1984 Nuclear Science Symp., to be published. [19] J.W. Mayer, in: Semiconductor Detectors, Eds. G. Bertoiini and A. Coche (North-Holland, Amsterdam, 1968) ch. 5.
Journal of Crystal Growth 70 (1984) 386—392 North-Holland, Amsterdam
TEMPERATURE GRADIENT SOLUTION ZONING GROWTH AND CHARACTERIZATION OF Zn~Cd1 ~Se SINGLE CRYSTALS -
A. BURGER and M. ROTH Graduate School of Applied Science and Technology, Hebrew University of Jerusalem, Jerusalem 91904, Israel
Single crystals of Zn~Cd1— ~Se with 0 ~ x ~ 0.3 have been grown from a high temperature Se solution using the TGSZ technique. 7 ohm cm) is analysis higher than thatand of CdSe ohm cm) grown by the same The Zn/Cd distribution has been measured using an original XRF system found crystals uniform (106 throughout any particular crystal. The darkdue resistivity Zn0 energy 3Cd07Se crystals (5 the x i0ternary compound. A discrete electron trapping level located 0.49 eV below the method, to the of larger band gap of conduction band with a trap density of 1.9 x 1011 cm has been attributed to a compensateddonor formed by an interstitial Cu impurity and a nearby Cd vacancy. The charge collection efficiency has been studied as a function ofvoltage applied to a thin Zn 0 3Cd0 7Se platelet. It is not limited, however, by the electron traps, but rather by the surface recombination of charge carriers. It has been suggested that an improved charge collection needed for the fabrication of (Zn,Cd) Se nuclear radiation detectors can be obtained by reduction of surface defects generated during the crystalline platelet processing.
1. Introduction The pseudobinary CdSe—ZnSe system is known to form a continuous series of solid solutions [1]. Single crystals of Zn~Cd1 ~Se have been grown from the vapor phase [2,31, and their optical properties have been investigated with respect to the application as laser screen materials in projection color 1’V. However, no information on the basic electrical properties of these ternary compounds exist in the literature. The charge transport properties of the Zn~Cd1 ~Se crystals are of particular interest, since the pure CdSe has been discovered recently [4,51as a novel semiconductor material suitable for fabrication of nuclear radiation detectors. In the present work we have grown for the first time single crystals of a ternary Zn0 3Cd07Se compound using the Temperature Gradient Solution Zoning (TGSZ) technique from the high temperature Se solution. This growth method has been chosen since in the case of CdSe [4] it yields relatively high dark resistivity (about 106 ohm cm) as-grown crystals. The vapor grown CdSe crystals, in contrast, exhibit much lower, typically 1 to 100 ohm cm, resistivity, which makes them unsuitable for lownoise room temperature operation as nuclear radia-
-
tion detectors [5]. Although thermal annealingofthe vapor grown CdSe in the Se2 vapor increase the resistivity by many orders of magnitude [5], the resulting reduction of the electron mobility, is due to the resistivity inhomogeneities, which degrade the charge transport properties of the material. The similarity of band structures and optical properties of CdSe and ZnSe [7], despite the difference in the crystal structure (hexagonal and cubic, respectively), suggests that the energy band gap (Eg) of the Zn~Cd1 ~Se must increase with the rising value of x. This trend has been experimentally verified for a complete series of solid solutions of a homologous Zn~Cd1 ~S compound [7]. Indeed, for the Zn0 3Cd0 7Se composition an experimental value of Eg = 2.00 eV has been determined [8], which is higher than Eg = 1.73 eV for CdSe (both measured at 300 K). Therefore, a lower dark current, beneficial for the detector operation, is expected at room temperature. The sufficiently high resistivity of the TGSZ grown Zn0 3Cd0 7Se crystals has allowed the study of their I—V characteristics, which have been analyzed in the framework of the space charge limited current (SCLC) theory [9]. The results have been compared with those obtained with the TGSZ CdSe crystals. Additionally, the charge collection efficiency in a thin
0022—0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
-
A. Burger, M. Roth
/
TGSZ growth and characterization of Zn~,,Cd
Zn0 3Cd07Se crystal has been studied using a standard nuclear measurements set up [10], and discussed in terms of the nuclear radiation detector performance.
2. Experimental 2.1. Single crystal growth The experimental set up used for the growth of single crystals was similar to that described previously for the TGSZ of CdSe. The starting materials were Alfa 5N CdSe and 5N ZnSe and the Aldrich 6N Se shot. The CdSe and ZnSe powders were further purified by sublimation in vacuum at 900 and 10000 C respectively. They were subsequently milled, mixed and sintered at 9000 C for 36 h. Typically 30 g charges were prepared with addition of 10 at% Se. Since neither the ternary nor the pseudobinary CdSe—ZnSe phase diagrams have ever been published, the liquidus temperature for the desired composition was determined experimentally using a conventional DTA set up. A 5 g charge of the Zn0 3Cd0 75e sintered compound with additional 10 at°,/0Se had been loaded into a 1 by 5 cm quartz tube and sealed under 10-i Torr vacuum. A similar reference tube with 3 g of Al203 powder had been prepared. Two identical Pt/Pt— 13% Rh thermocouples were inserted into wells made at the bottom ends of the sample and reference tubes. The differential temperature recording was carried out using a double pen Kipp and Zonen recorder directly, with a 20 1tV full scale. The thermal arrest temperature of the liquid (melted sample) was determined on slow cooling C/min), which provided the necessary close to equilibrium conditions of the experiment. The liquidus temperature of the Zn0 3Cd0 7Se + 10 at % Se composition was found to be 1120°C with a reproducibility of ±2°C. Therefore, prior to crystal growth the melt was soaked at a temperature of 1150°Cfor 24 h. During the crystal growthrun the ampule was lowered in a temperature gradient of 20°C/cmat a rate of 1 cm/day. Using these growth conditions, 1.5 cm diameter and several cm long single crystals of Zn0 3Cd0 7Se could be routinely produced. (30
3— ~Se
387
2.2. Sample preparation Slices with thicknesses varying from 0.2 to 2 mm were either cut (using a wire saw) or cleaved from the single crystal ingots.They were further thinned down through mechanochemical lapping and polishing in a 3 bromine—methanol solution. Additional etching in a 10 0/0 bromline—methanol solution followed by rinsing with pure methanol and triply distilled water was carried out before the application of contacts. Good ohmic contacts, necessary for the observation of space charge limited currents, could be produced using the In-Hg eutectic alloy. In the case of nuclear measurements blocking contacts were applied, usually by painting the carbon based Aquadag solution, though evaporated gold electrodes produced similar results. 0/~
2.3. Composition analysis The axial and radial Zn/Cd distributions were studied using a specially designed X-Ray Fluorescence (XRF) analysis system shown schematicallyin fig. 1. A 5 mCi gamma-ray point source of 125J served to excite the characteristic X-ray emission lines of Zn(Kct, 9.6 keY), Cd(Kcç 23 keV and KfI, 26 keY) and Se(Kc~,11.1 keY). The X-ray radiation spectrum was registered using an 8% energy resolution Hg12 detector [11] and displayed on the screen of an ORTEC Model 7100 multi-channel analyzer. A commercial Link XRF analysis system in conjunction with the JEOL JSM 35 scanning electron microscope were used for initial calibration of the composition of the Zn0 3Cd0 7Se crystal. A small sample was cut from the bottom part of the crystal for the purpose of calibration, and the Zn/Cd ratio was found to correspond exactly to the molar ratio of ZnSe and CdSe charges introduced into the growth ampoule. 3. Results and discussion The maximal concentration of ZnSe in the Zn~Cd1 Se crystals grown in the present work corresponded to x = 0.3. The growth of crystals of composition with x> 0.3 was limited by the equipment used. A photograph of a typical Zn0 3Cd0 7Se crystal grown by the TGSZ method is shown infig. 2 -
388
A. Burger, M. Roth
/
TGSZ growth and characterization of Zn~Cd,- ~Se
HV Power Supply
Crystal
Hgli Detector
~
125)
~ Preamplili!jj Amplltier
L{
MCA
Source
Fig. I. X-ray fluorescence analysis system based on the Hg1
2 nuclear radiation spectrometer.
Fig. 2. Photograph of a single crystal of Zn0 3Cd0 7Se (left) grown by the TGSZ technique and the polycrystalline residue (right) of the starting materials.
together with the polycrystaffine residue of CdSe, ZnSe and Se remaining in excess after the crystallization of Zn0 3Cd07Se. As in the case of CdSe TGSZ growth [4] the homogeneity of the crystal has been achieved by lowering the ampoule through the temperature gradient existing in the furnace, which has kept the growth interface at a constant temperature. The lowering speed is limited by the solution zone motion speed, or by the upward Se diffusion rate in the molten zone. It has been verified experimentally that only polycrystalline boules of Zn0 3Cd0 7Se could be produced at lowering speeds faster than 1 cm/day. The crystal shown in fig. 2 was cut with a wire saw and the Zn/Cd atomic ratio was measured using the Hg12 XRF spectrometer system described in the previous section. The surprising result, which is very important for future applications of the mixed (Zn, Cd) Se crystals, was that both axial and radial ZnSe/CdSe molar ratio’s were found constant within the statistical error of 0.6%. This suggests that the solubilities and the diffusion rates ofZnSe and CdSe
in the Se solvent are very similar. The absence of any apparent segregation in the process of solidification may also indicate that the solidus—liquidus tie-lines of the Zn—Cd—Se system correspond to constant Zn/Cd ratios, but any certain confirmation of this hypothesis can come only from future studies of the ternary phase diagram. Another result observed using the LINK XRF analysis system, i.e. that the Zn/Cd ratio in the crystal corresponds exactly to the molar ratio of ZnSe and CdSe introduced into the feed material, indicates that the solidus—liquidus gap of the ZnSe—CdSe pseudobinary system is very small, at least for the Zn0 3Cd0 7Se composition. It is well known that CdSe crystallizes in the wurzite (hexagonal) structure, while ZnSe in the zincblende (cubic) structure. The structure of the solid solutions, Zn~Cd1 ~Se, depends on the cornposition of the compound. It has been found to the hexagonal for 0 ~ x ~ 0.5, as follows from the X-ray diffraction analysis performed by Budennaya et al. [3]. This result has been confirmed by our own X-ray diffraction measurements carried out using -
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TGSZ growth and characterization of Zn~Cd,— ~Se
the Zn03Cd07Se crystals. We have found that the crystals can be easily cleaved along the (1120) planes.samples In some of wire sawn, havecases been cleaved, prepared instead for the electrical measurements. The TGSZ grown Zn 0 3Cd0 7Se crystals exhibit a relatively high dark resistivity. A mean value of = 5 x iO~ohm cm has been obtained using the four point probe method [12]. Astointhethelack case of CdSe [4] the high resistivity is due of Se vacancies in crystals grown in an environment of excess Se. TheresistivityoftheZn 03Cd07Secrystals is high enough to use the SCLC theory [13] for the analysis of the charge transport properties of the crystals. Fig. 3 shows a typical I—V curve taken with a 1.8mm thick Zn02 3Cd0 sample at room temperaarea7Se metal contacts have been ture. Three applied to themm crystal, one of them being ohmic. The current rises linearly at low voltages, up to about 30 V. At higher voltages a clear quadratic increase of the current is observed, indicating that a discrete trap level lunits the current flow in this region. A steepnse of the current corresponding to the trap-filled-limit
389
(TFL) regime occurs at VTFL = 600 V. The narrow superquadratic transition region between the square 2) law behaviorand the onset ofthe TFL is not (Icc V extended enough to claim that a “diffused” rather than a discrete trap level is involved. The density of traps (N8) for one-carrier injection can be derived from a simple relation given by the SCLC theory: 2N ‘2 1 ~TFL ea ~, where e is the electron charge, a is the sample — —
thickness and t~is the static dielectric constant [7]. The dielectric constants of CdSe and ZnSe are very similar and a mean value of ~ = 9 is adopted for the present calculations. Thus, a value of 1.9 x 1011 cm ~ obtained forthe trap density (NJ. trap level energies (P28) have beenthe derived from theThe experimental I—V curve, since quadratic region follows Child’s law [9]: -
‘
—
8
A
£1T7213
lieU V
where A is the contacxt area, ~i is the electron (for an n-type crystal) mobility and 0 is the ratio of the free electron density to the trapped electron density in the crystal: ,
0
=
n/n 8
=
(N0/gN8) exp(—E8/kT),
(3)
where N0 is the effective density of states in the conduction band and gis the degeneracy factor equal to 2 for the electron traps [14].
:
The values of /ie and N0 are material dependent, and they are not known either for Zn0 3Cd07Se nor for the zincblende structure ZnSe. Therefore, s’ and 3 of pure CdSe2Y~ are taken for N the = 720 cm 0 =approximate 1.15 x 1018 evalucm— ation of the trap level energies. The value of P2 8 = 0.49 eV has been obtained by solving eqs. (2) and (3) and using the parameters described above. Table 1 summarizes the electron trap parameters of
10 3 .. 102
•.
/
101 Table 1 Trapping levels and concentrations 100 100
I
101 V
I
(V) 102
3) I
io~
2. taken with Fig. 3. TypicalI—V curve ofthe Zn0 3Cd07Se crystal a 1.8 mm thick platelet and contact area of 3 mm
Crystal TGSZ Zn
E,(eV)
N8(cm
0 3Cd0 75e 0.49 1.9 x 10” _____________________________________________________ TGSZ CdSe [4] 0.51 1.0 x 1012
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TGSZ growth and characterization of Zn~Cd,- ~Se
the TGSZ grown Zn0 3Cd07Se and the TGSZ grown CdSe crystals for comparison. Both the trap level energies and the trap densities are apparently similar, especially in view of the rather approximate nature of our calculations, and of course, the differences in the crystal compositions. Manfredotti et al. [15] have also reported on the existance of an electron trap with an energy of 0.49 eY below the conduction band and density of 3 x 1011 cm in CdSe crystals with a mean dark resistivity of 108 ohm cm at room temperature. It is interesting to as-grown, note that medium this kindresistivity of trap crystals usually appears in the and we have attirbuted it to the formation of Cu impurity induced Frenkel-type defects, namely Cu 1—Vcd pairs. It would be, therefore, interesting to grow single crystals of CdSe and CdSe—ZnSe solid solutions by the TGSZ method using Cu free starting materials. The latter must be sinthesized from the constituent elements, since Se is known to be the major source of the Cu impurity. Preliminary experiments show that copper can be extracted from Selenium very effectively using the zone-refming method. Although the TGSZ growth minimizes the amount ofSe vacancies, a small amount ofvacancies remain in the crystals and they are responsible forthe residual n-type conductivity of the TGSZ grown Zn0 3Cd07Se and CdSe [4]. The n-type conductivity associated with the Se vacancies [6] is much lower in the TGSZ crystals than in the crystals grown from the vapor phase using a stoichiometric starting material [5], since the former are grown in a Se-excess environment. High temperature Se annealing of CdSe crystals [16] or vapor growth in the Se-excess atmosphere [17] can further reduce the number of Se vacancies, or donor levels, converting the crystals into nearly intrinsic or even p-type conductive. In other words, the resistivity of the crystals is closely related to the amount of Se vacancies present. It is, therefore, surprising that the TGSZ grown Zn0 3Cd0 7Se crystals exhibit a higher resistivity than the CdSe crystals grown by the same method. The opposite could be expected, since the Zn0 3Cd0 7Se crystals are grown at a temperature higher by several tens of degrees, allowing for a higher concentration of Se vacancies to be formed. The simplest explanation that can be given ~
is that the net concentration of ionized donors (ND NA) is much smaller than the concentration of thermally induced electrons. Thus, the resistivity depends mostly on the energy bandgap of the material, being higher for Zn0 3Cd0 7Se due to the larger bandgap of this material. As a consequence, lower leakage current and thus, lower electronic noise detectors (in comparison with CdSe) can be fabri. cated from the TGSZ grown Zn0 3Cd0 7Se crystals. This has been proven experimentally in a nuclear measurement using electronic system shown in 55Fean(5.9 keY) gamma point source fig. 1, butofwith instead the asample. The 5.9 keY gamma-peak could be clearly resolved from the noise threshold, which was impossible before with CdSe detec—
tors [4]. A detailed description of the Zn0 3Cd07Se detector performance will be given elsewhere [18], however some results of the nuclear measurements concerning the material dependent parameters are reported here. In a nuclear measurement carried out using a semiconductor detector gamma-rays (or X-rays and charged particles in other cases) interact with the crystal by converting most of their energy into dcctron—hole pairs. The charge carriers are collected by an applied electric field (E), and the resulting current pulse is proportional to the radiation energy. Usually, the signal is amplified and recorded as a histogram of the observed radiation events giving the number ofevents in each energy interval. In our case, the histograms or spectra, have been recorded using a multichannel analyzer. Thus, the total charge collected at the electrods is:
Q
=
~ iN~,
(4)
where i is the channel number and N~is the number of events in the ith channel. Q is a complex function of the mean drift length 2 = /iEE, where ji is the mobility and t is the trapping time of the charge carriers. At high electric fields the collected charge reaches the saturation value Q0, and the charge collection efficiency ~ for a uniform field * and the
*
Since the Zn0 3Cd0 7Se as well as the CdSe detectors never show the polarization effect, no space charge is beingbuilt up in the detector, which might alter the internal electric field.
A. Burger, M. Roth
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TGSZ growth and characterization of Zn~Cd
1- ~Se
electron transport only* is given by the modified Hecht equation [10]: =
+
=
Qo
\
—~_i ~-1~-~ [~ exp(~T_~l, —
lieEJ
L
L
\lietE’JJ
(5) where L is the thickness ofthe detector and S is the carrier Q and Qsurface recombination velocity. The values of 0 are determined from the experiment using eq. (4). The experimentally dependence of the collection efficiency on obtained the magnitude of the applied electric field is shown in fig. 4. The striking result here is that the full charge collection is achieved at very high fields, above 4 x io~ V/cm. Even with mercuric iodide [10] having a much lower electron mobility, weak fields are needed for the full charge collection. There are two major factors that may limit the charge collection in the present case, namely the surface recombination and the bulk trapping of the charge carriers. A convenient estimate of the parameters involved can be made using the haif-maximum-collection value ofthe electric field (Ehmc). The latter can be derived from eq. (5), for ~ = 1/2, and is given by Eh mc
=
S//ie
+
(0.63//i e t)L.
(6)
___________________________________________ 100
A A
~
80 60
A A
A
A
A
A
A
40
A
20 A
A
00
3 (V/cm) Fig. 4. Charge collection ELECTRICefficiency FIELD as -x10 a function of the applied
391
We know from the SCLC analysis of the I—V curves described above that a single 3kindexists of trap in with our aZn density N8 = 1.9 x lO~cm 03Cd07Se crystals. The trapping time (t) can be then evaluated from the well known relation [19]: r= (NtaeVth)’, (7) where V~his the thermal velocity electrons being 0e is the of electron trapping equal to —~107 cm/s and cross section. Assuming even the largest cross section ever found in compound 2, we obtain t semiconduc5 x 10-6 s. tors [19], i.e. 1013 cm Using this value together with lie = 720 cm2 Y ‘ s 1, L = 0.01 cm (thickness of the detector used in this experiment) and Ehmc = 1600 V/cm (from fig. 4), we now calculate the carrier surface recombination velocity S = 106 cm/s. Comparing the calculated value of S with the much smaller ratio L/r = 2 x 10~cm/s, it is clear from eqs. (5) and (6) that the charge collection efficiency depends entirely on the surface recombination kinetics of the carriers. Even for several mm thick detectors instead of the 0.1 mm thick one used in the present work the condition L/z ~ S would be valid. The origin of the high carrier surface recombination velocity is still unclear. The most probable hypothesis is, however, that a high concentration of surface defects is introduced by the surface treatment during the detector fabrication. It is evident that no bulk defects ofthe Zn 0 3Cd0 7Se crystals play a role in the observed phenomenon. The charge collection properties of the CdSe crystals have not been studied before [4], since it was impossible to extract the 5.9 keY gamma-ray peak from the dcc‘~
—
.
tronic noise background. However, a similar surface damage in the fabrication process of the CdSe detectors occur.defects It is, therefore to study the may surface of both plausible CdSe and Zn~Cd 1 ~Se crystals in correlation with the surface —
electric field measured 55Fe (5.9 with keY) a 0.1gamma-ray mm thick point Zn0 3Cd07Se source through crystal irradiated the negative with electrode. a *
The penetration depth of the 5.9 keY gamma radiation for the Zn 0 3Cd0 7Se crystal is only 3—4 ~m which is much smaller than the detector thickness used in this work. Therefore, the contribution of the hole transport induced charge is negligible, when the polarity of the irradiated electrode is negative,
processing. 4. Conclusion
Large single crystals of the Zn~Cd1 ~Se ternary compound with x up to 0.3 have been grown usmg the TGSZ technique. Both the axial and radial distri—
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A. Burger, M. Roth
/
TGSZ growth and characterization of Zn~Cd
butions of the cation have been found uniform throughout the crystals provided that the growth interface is kept at a constant temperature. The TGSZ growth based on crystallization from Se excess solution yields high dark resistivity 5 x l0~ohm cm) Zn0 3Cd0 7Se crystals aJmost free of Se vacancies, which are responsible for the n-type conductivity. The resistivity has been found to depend mostly on the thermally generated carriers. Therefore, the resistivity of Zn0 3Cd0 7Se crystals is higher by an order of magnitude than that of the CdSe, due to a larger energy bandgap. Lower leakage current and thus, lower noise detectors can be fabricated from these crystals. The electron transport in the Zn0 3Cd07Se crystals has been found to be limited by a discrete trap level located at 0.49 eV below the conduction band. The nature of the trap level is found to be similar to that observed in the CdSe crystals, and is related to the production of pairs including a Cu interstitital impurity (donor) compensated by a Cd vacancy. However, the performance of Zn~Cd1 ~Se crystals as nuclear radiation detectors has been found not to be limited by the bulk transport properties of the charge carriers, but rather by the high carrier surface recombination velocity originating probably from surface defects. (‘—j
—
1- ,~Se
References [1] S. Forgue, R. Goodrich and A. Cope, RCA Rev. 12 (1951) [2] Reimers, Phys. Status Solidi 35 G.S. (1969)Pekar 707. and G.N. [3] P. L.D. Budennaya, A.!. Nizkova, Polisskii, Inorg. Mater. 18 (1982) 760. [4] A. Burger and M. Roth, J. Crystal Growth 67 (1984) 507. [5] A. Burger, I. Shilo and M. Schieber, IEEE Trans. NucI. Sci. NS~30(1983) 368. [6] AL. 5280. Robinson and RH. Bube, J. Appl. Phys. 42 (1971) [7] H. Hartman, R. Mach and B. Selle, in: Current Topics in Materials Science, Yol. 9, Ed. E. Kaldis (North-Holland, Amsterdam, 1982). [8] Y. Brada, private communication. [9] M.A. Lampert, Phys. Rev. 103 (1956) 1648. [10] A. Levi, M. Schieber and Z. Burshtein, J. Appl. Phys. 54 (1983) 2472. [11] J. Nissenbaum, A. Holzer, M. Roth and M. Schieber, Advan. X-Ray Anal. 24 (1980) 303. [12] H.H. Weider, in: Non-Destructive Evaluation of Semiconductor Materials and Devices, Ed. J. Zemel (Plenum, New York, 1979). [13] MA. Lampert, Phys. Rev. 103 (1956) 1648. [14] J.S. Blakemore, Semiconductor Statistics (Pergamon, Oxford, 1962). [15] C. Manfredotti, A. Rizzo, L. Vasanelli, S. Galassini and L. Ruggiero, J. AppI. Phys. 44 (1973) 5463. [16] M. Itakura and H. Toyoda, Japan. J. Appl. Phys. 4 (1965) 560. [17] R. Baubinas, Z. Janushkevichus and A. Sakalas, Mater. Res. Bull. 8 (1973) 817. [18] A. Burger, M. Roth and M. Schieber, in: Proc. 1984 Nuclear Science Symp., to be published. [19] J.W. Mayer, in: Semiconductor Detectors, Eds. G. Bertoiini and A. Coche (North-Holland, Amsterdam, 1968) ch. 5.